In recent years, awareness of ecological problems is growing on a worldwide scale. Especially, the deepening concern for global warming by CO2 emission is increasing the demand for clean energies more than ever. At the moment, a solar battery is expected as a clean energy source because of its safety and good manageability.
Typical structures of a photovoltaic element used in a solar battery are as follows.
As the first structure, one power generation layer 101 is formed, and electrodes 102 and 103 are attached to both surfaces of the power generation layer to output electricity, as indicated by the sectional view shown in FIG. 10. Various materials are used for the power generation layer 101, including single-crystal silicon, polysilicon, crystallite silicon, amorphous silicon, and a compound semiconductor. This structure needs to have only one power generation layer and can be formed with a small number of manufacturing processes.
A transparent conductive layer is normally used as the electrode 102 on the light-receiving surface side. A metal electrode bus is sometimes used partially. The other electrode 103 has the same structure as that of the electrode 102. Alternatively, the electrode 103 may have a metal conductive layer on the entire surface or use a metal substrate as the metal conductive layer.
The output from the power generation layer 101 is supplied to a load 21 through the electrodes 102 and 103.
As the second structure, two power generation layers 111 and 112 are stacked and electrically connected in series with each other, and electrodes 113 and 114 are attached to the surfaces of the two power generation layers to output electricity, as indicated by the sectional view shown in FIG. 11.
The same material as that in the first structure can be used for the two power generation layers 111 and 112. However, when materials having different optical wavelength sensitivities are used for the power generation layers 111 and 112, the optical spectrum utilization efficiency increases, resulting in an increase in conversion efficiency. The two power generation layers 111 and 112 are connected in series. For this reason, the current balance between the two power generation layers 111 and 112 is designed not to cause IV mismatch. The electrodes 113 and 114 are the same as in the first structure.
The output from the power generation layers 111 and 112 connected in series is supplied to the load 21 through the electrodes 113 and 114.
FIG. 11 shows an example with two power generation layers. A structure having three or more power generation layers which are stacked and electrically connected in series to output electricity is also known.
As the third structure, two power generation layers 121 and 122 are formed, electrodes 123 and 124 are formed on both surfaces of the power generation layer 121 while electrodes 125 and 126 are formed on both surfaces of the power generation layer 122, and an insulating layer 127 is inserted between the electrodes 124 and 125, as indicated by the sectional view shown in FIG. 12. The power generation layers, 121 and 122 are electrically insulated from each other by the insulating layer 127.
The material of the two power generation layers 121 and 122 is the same as in the second structure. As in the second structure, when materials having different optical wavelength sensitivities are used, the optical spectrum can be effectively used. In the third structure, however, the two power generation layers 121 and 122 can be independently used because they are not electrically connected in series. Since no IV mismatch occurs, the current balance between the two power generation layers 121 and 122 need not be taken into consideration.
The uppermost electrode 123 and lowermost electrode 126 are the same as in the first structure. Each of the electrodes 124 and 125, which are located on both surfaces of the insulating layer 127, has a transparent conductive layer or may be designed to partially use a metal electrode bus, like the electrode 102 of the first structure. However, the design needs to take account of stacking the insulating layer 127 and the power generation layers 121 and 122.
The output from the power generation layer 121 is supplied to the load 21 through the electrodes 123 and 124. The output from the power generation layer 122 is supplied to a load 22 through the electrodes 125 and 126. That is, the outputs from the two power generation layers are supplied to the separate loads.
As the fourth structure, two power generation layers 131 and 132 are formed, electrodes 133 and 134 are formed on both surfaces of the power generation layer 131, and the electrode 134 and an electrode 135 are arranged on both surfaces of the power generation layer 132 such that the electrode 134 is shared by the two power generation layers 131 and 132, as indicated by the sectional view shown in FIG. 13. The power generation layers 131 and 132 are electrically connected in series (Japanese Patent Laid-Open No. 57-153478).
The output from the power generation layer 131 is supplied to the load 21 through the electrodes 133 and 134. The output from the power generation layer 132 is supplied to the load 22 through the electrodes 134 and 135. That is, the outputs from the two power generation layers are supplied to the separate loads.
The material of the two power generation layers 131 and 132 is the same as in the second and third structures. As in the second and third structures, when materials having different optical wavelength sensitivities are used, the optical spectrum can be effectively used. In the fourth structure, however, the two power generation layers 131 and 132 can be connected to separate loads and independently used although they are electrically connected in series. Since no IV mismatch occurs, the current balance between the two power generation layers need not be taken into consideration. In addition, unlike the third structure, one electrode and one insulating layer between the two power generation layers can be omitted.
The above photovoltaic elements however have the following problems.
In the first structure, since only one material is used for the power generation layer, the optical wavelength sensitivity is limited, and the optical spectrum cannot be effectively used.
In the second structure, if the current balance between the two power generation layers should be ensured, each power generation layer cannot always effectively use the optical spectrum.
In the third structure, since the two electrodes that come into contact with the both surfaces of the insulating layer 127 must be formed to have a sufficiently low resistance with respect to the generated current, the transparent conductive layer of each of the electrodes 124 and 125 must be thick. The transmission loss of a transparent conductive layer is not 0%, though it is transparent. Hence, when the transparent conductive layer becomes thick, the light amount to the power generation layer 122 on the lower side decreases. In addition, to form a thick transparent conductive layer is expensive. When a metal electrode bus is partially used, the transparent conductive layer can be thinned to some extent. Even in this case, however, the decrease in light amount and the increase in cost due to the thick transparent conductive layer are inevitable. The presence of the insulating layer 127 also poses the problems of smaller light amount and higher cost.
In the fourth structure, no insulating layer is necessary between the two power generation layers, and only one common electrode suffices between them, unlike the third structure. Hence, the decrease in light amount can be suppressed, and the cost can be reduced. However, since the two power generation layers are electrically connected in series, only loads that are usable in an electrically serial state can be used. In addition, since the two power generation layers are electrically connected in series, only a low voltage corresponding to one power generation layer can be supplied to the load. Since usable loads are limited, the application range is limited, too. Furthermore, since the feed voltage to the load is low, the wiring loss tends to be large.